1. Field of the Invention
The present invention relates to an apparatus and method for calibrating an extreme ultraviolet (EUV) spectrometer, and more particularly, to an apparatus and method for calibrating an EUV spectrometer to accurately measure a wavelength of a spectrum of EUV light used for EUV lithography and mask inspection technology.
2. Discussion of Related Art
In general, an EUV beam, for example, electromagnetic radiation (also known as soft X-rays) having a wavelength of about 124 nm or less, which includes light having a wavelength of 13.5 nm, can be used in a photolithography process to form a very small pitch on a substrate, for example, a silicon wafer.
That is, EUV light and X-rays are located in a shorter wavelength region than visible light, and thus can enhance measurement resolution according to a diffraction limit which limits sizes of wavelengths in precision measurement using light, and can be used for fine measurement or nondestructive testing involved in biotechnology using a good transmission characteristic by extending to the X-ray region.
Particularly, if a good coherent light can be generated at the same time, various applications using interference and diffraction phenomena of light are possible. Because a repetition rate of an incident femtosecond laser can be maintained, it can be used for precision spectroscopy or frequency standard measurement, and so on in EUV and X-ray regions.
One of various methods of generating EUV light and X-rays is a method using a synchrotron. When EUV light and X-rays are generated using a synchrotron, there are advantages in that a large amount of light of good quality can be obtained and various wavelength bands can be obtained at the same time, however, because a facility itself is very enormous and expensive, there is a problem in that it cannot be simply configured in a laboratory stage.
As a method of overcoming this problem, recently, a high-order harmonic generation (HHG) method using a high power femtosecond pulse laser has been proposed, and thus coherent EUV light and soft X-rays can be generated with a relatively small experimental device.
In the HHG method, electrons are ionized, move along a track and are recombined by applying a high time-varying electric field to an inert gas such as, for example, argon (Ar), neon (Ne), xenon (Xe), and so on, and the energy corresponding to the sum of the ionization energy and kinetic energy of the electrons generates light of the EUV and X-ray band.
HHG has typically been designed or made by injecting an inert gas into a gas cell, with the used inert gas leaving the gas cell naturally.
Alternatively, lithium (Li), tin (Sn) and a semiconductor device are also able to generate EUV light in addition to inert gases such as Ar, Ne, Xe, and so on, and inert gases are used to generate EUV light in current HHG technology using a gas cell only because HHG using a gas cell uses a gas as a medium. Thus, there is no specific limitation to inert gases, and EUV light can be generated using other methods.
Meanwhile, because ultra-refinement of a semiconductor process for high integration is required, light sources used for lithography including G-line (436 nm), I-line (365 nm), krypton fluoride (KrF) (248 nm) and argon fluoride (ArF) (193 nm) have been developed.
However, new lithography technology extending resolution in semiconductor processes of 90 nm or less was still required, for which ArF immersion lithography and double-patterning lithography were developed, and thus dynamic random access memories (DRAMs) and NAND flash memories have recently been successfully mass-produced in 23 nm and 20 nm processes, respectively.
Among various techniques being studied to surpass the ArF lithography as next generation lithography, EUV lithography has reached a stage of trial production in major semiconductor companies, and research and development thereof are being actively conducted worldwide.
Meanwhile, in order to inspect defects of mask made by EUV lithography, EUV light is also needed. Particularly, a technique of inspecting defects of a mask with the same wavelength as used in lithography is called an actinic mask inspection technique. In order to generate EUV light with the same EUV wavelength as used in lithography, the HHG method is needed. In order to check whether EUV light generated by the HHG method matches a desired wavelength, a spectrum should be measured using an EUV spectrometer. In this case, calibration of the spectrometer is necessary in order to accurately measure the spectrum. As a calibration method of EUV light, there is a method using an atomic line which has been used for some time. Each inert gas has unique fluorescence lines known as atomic lines. Because wavelength values of atomic line spectra have been studied and widely known since decades ago, EUV light can be calibrated based on a position and a wavelength value of an atomic line spectrum.
However, in order to acquire a degree of strength that a charge-coupled device (CCD) camera of the spectrometer is able to measure, energy several times greater than energy of a laser used for HHG is needed. Therefore, there is a problem in that a laser light source that outputs energy greater than that of a laser for generating EUV high-order harmonics is necessary.